The present invention relates to a strip intended to be combined between a vehicle bodywork and glazing with a view to forming an acoustic damping means to absorb the vibration waves transmitted through the glazing and bodywork of the vehicle.
Such a strip may be used in particular for vehicle, particularly motor vehicle, lazing, with a view to improving the acoustic comfort of its interior.
In a motor vehicle, the sources of annoyance of mechanical, thermal, visibility, etc. origin have gradually been overcome. However, improvement in acoustic comfort is still very much an ongoing concern.
Aerodynamic noise, that is to say noise created by the friction of the air over the moving vehicle, has, at least in part, been dealt with at its source, that is to say that in order to save energy, the shapes have been modified, thus improving penetration through the air and reducing the turbulence which itself is a source of noise. Among the walls of a vehicle that separate the source of external aerodynamic noise from the interior space where the passenger is situated, the windows are obviously the most difficult to deal with. It is not possible to use pasty or fibrous absorbents that are reserved for the opaque walls, and for practical or weight reasons, the thicknesses cannot be increased without due consideration. European patent EP-B1-0 387 148 proposes glazings which provide good insulation against noise of aerodynamic origin without their weight and/or thickness being excessively increased. The patent thus proposes laminated glazing in which the interlayer has a flexural damping factor υ=Δf/fc of greater than 0.15, the measurement being carried out by exciting, by means of a shock, a laminated bar 9 cm long and 3 cm wide made of a laminated glass in which the resin is between two panes each 4 mm thick and by measuring fc, the resonant frequency of the first mode, and Δf the width of the peak at an amplitude A/√2 where A is the maximum amplitude at the frequency fc such that its acoustic attenuation index does not differ, for any of the frequencies above 800 Hz, by more than 5 dB from a reference index increasing by 9 dB per octave up to 2000 Hz and by 3 dB per octave at the higher frequencies. In addition, the standard deviation σ of the differences in its acoustic attenuation index with respect to the reference index remains lower than 4 dB. The thicknesses of the two glass panes may be identical and equal to 2.2 mm. That patent thus proposes a general solution to the problem of acoustic insulation of a vehicle against aerodynamic noise.
By contrast, the treatment of glazing against solid-borne noise, that is to say against noise transmitted via solid bodies and in the frequency domain from 50 to 300 Hz or even 800 Hz, is more difficult to achieve. This is because it turns out that the use of connecting pieces is not enough to avoid the transmission of noise by vibration of the glazing. Indeed it has been found that at certain engine speeds, a humming noise perceivable by the passenger occurs and thus causes a source of annoyance. What happens is that the turning-over of the engine creates vibrations which are transmitted, for example, to the bodywork and thus, through a chain effect, to the glazing. It is known that the energy acquired by an object subjected to a shock generates a vibration phenomenon and that, immediately after the shock, the object that has become free again vibrates in its natural mode. A vibrational frequency is associated with each mode. The amplitude of the vibration depends on the initial excitation, that is to say on the spectral component of the shock (the amplitude of the shock at the frequency studied) and on the area of impact of the shock, the modal deformation being greater or smaller according to whether the shock occurs at a vibration antinode or at a vibration node.
For a natural mode to be excited, it is necessary:
(1) for the deformation caused at the point of impact not to be situated on a vibration node of the mode, and
(2) for the energy spectrum of the shock to have a component at the resonant frequency of the mode.
The latter condition is practically always satisfied because a very brief shock exhibits a practically uniform energy spectrum.
The first condition is also satisfied, and, for a bar that is free at both ends, for example, all that is required is for one of the ends to be attacked and all the flexural modes will be excited.
Solid-borne excitation is peripheral and it has been demonstrated that, at certain vibrational frequencies of the engine, that is to say at certain engine speeds, at least one of the glazings has a vibration mode and the cabin of the vehicle has an acoustic mode, the coupling between these two modes amplifying the hum resulting from the acoustic radiation via the glazing of the energy originating, in this instance, from the engine. Of course, engine speed that gives rise to these phenomena is specific to each type of vehicle and cannot thus be generalized to a single value.
Hence, in order to improve acoustic comfort in the cabin of the vehicle with respect to solid-borne noise, patent EP 0 844 075 proposes laminated glazing comprising at least one interlayer film possessing very satisfactory damping qualities as regards solid-borne audible noise because it has a loss factor tan δ greater than 0.6 and a shear modulus G′ of less than 2·107 N/m2, in a temperature range between 10 and 60° C.
A solution that may be an alternative or a supplement to the use of glazing with an acoustic damping property may be to combine around the periphery of the glazing a strip that has an acoustic damping property. Patent application WO 04/012952 discloses a strip which, in order to provide such an acoustic damping property, has to have a real equivalent stiffness per unit length K′eq at least equal to 25 MPa, combined with an equivalent loss factor tan δeq at least equal to 0.25.
The equivalent stiffness per unit length is the equivalent stiffness of the strip per linear meter of strip.
The equivalent strip stiffness is the stiffness of the entirety of the strip irrespective of the number of materials or their constitution.
The stiffness is a quantity that relates the deformations of the strip to the loadings applied to it. The stiffness is defined by the rigidity of the materials that make up the strip and by the geometry of the strip, the rigidity being a quantity characteristic of the material and dependent on the Young's modulus and/or the shear modulus. The Young's modulus is related to the stresses and deformations experienced by the material when working in tension-compression, while the shear modulus is related to the stresses and deformations experienced by the material when working in shear.
The equivalent loss factor tan δeq is the loss factor of the entirety of the strip regardless of the number of materials and their constitution.
The loss factor is defined by the ratio between the dissipative power, that is to say the conversion of the energy of deformation of the strip into heat energy throughout the strip, and the stiffness per unit length.
In order to determine the real equivalent stiffness per unit length K′eq and the equivalent loss factor tan δeq of a strip made up of one or more materials, these quantities are estimated using a visco-analyzer. The visco-analyzer measures the real equivalent stiffness K′eq and the equivalent dissipative power K″eq of a specimen of strip of a cross section identical to that of the strip and of a length L then the following calculations are carried out:                the ratio between the measured real equivalent stiffness and the length L of the strip in order to obtain the real equivalent stiffness per unit length K′eq of the strip;        the ratio between the measured equivalent dissipative power and the measured real equivalent stiffness in order to obtain the equivalent loss factor tan δeq of the strip.        
In this type of strip mentioned hereinabove only stresses and deformations experienced by the material when working in tension-compression in the direction normal to the glazing are taken into consideration work in shear being negligible. This is because the bodywork is so rigid by comparison with the strip that it does not deform and cannot absorb vibrational energy. Only the strip deforms significantly and dissipates mechanical energy by working mainly in tension-compression.